1. Field of the Invention
The present invention relates to a flux guide read head with an in stack biased CPP sensor and, more particularly, to such a sensor wherein a bias stack is located in the sensor stack for longitudinally biasing a free layer of the sensor.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a tunnel junction sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a tunneling barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The tunnel junction sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers or first and second lead layers, are connected to the tunnel junction sensor for conducting a tunneling current therethrough. The tunneling current is conducted perpendicular to the major thin film planes (CPP) of the sensor as contrasted to a spin valve sensor where a sense current is conducted in (parallel to) the major thin film planes (CIP) of the spin valve sensor. In addition, a spin valve sensor can be set up in a CPP geometry where the current is conducted perpendicular to the plane. Although the description here is for the tunnel junction sensor, the art of the invention can be applied to CPP spin valve sensors. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the tunneling current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. The sensitivity of the tunnel junction sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance to maximum resistance and R is the resistance of the tunnel junction sensor at minimum resistance.
The first and second shield layers or first and second lead layers may engage the bottom and the top respectively of the tunnel junction sensor so that the first and second lead layers and/or the first and second shield layers conduct the biasing current through the tunnel junction sensor perpendicular to the major planes of the layers of the tunnel junction sensor. The tunnel junction sensor has first and second side surfaces which are normal to the ABS. The prior art has the first and second hard bias layers abut the first and second side surfaces respectively of the tunnel junction sensor for longitudinally biasing the magnetic domains of the free layer. This longitudinal biasing magnetically stabilizes the free layer and maintains the magnetic moment of the free layer parallel to the ABS when the read head is in the quiescent condition. In our invention, the longitudinal bias field is provided by an in-stack bias stack.
Magnetic head assemblies, wherein each magnetic head assembly includes a read head and a write head combination, are constructed in rows and columns on a wafer. After completion at the wafer level, the wafer is diced into rows of magnetic head assemblies and each row is lapped by a grinding process to lap the row to a predetermined air bearing surface (ABS). In a typical tunnel junction read head all of the layers of the read head are exposed at the ABS, namely first edges of each of the first shield layer, the free layer, the barrier layer, the pinned layer, the pinning layer and the second shield layer. Opposite edges of these layers are recessed in the head. The barrier layer is a thin layer, on the order of 5 xc3x85-30 xc3x85, which places the free and pinned layers very close to one another at the ABS. When a row of magnetic head assemblies is lapped there is a high risk of magnetic material from the free and pinned layers being smeared across the ABS to cause a short therebetween. Accordingly, there is a strong-felt need to construct magnetic head assemblies with tunnel junction heads without the risk of shorting between the free and pinned layers at the ABS due to lapping.
A scheme for preventing shorts across the barrier layer of the tunnel junction sensor is to recess the tunnel junction sensor within the head and provide a flux guide between the ABS and the sensor for guiding flux signals from the rotating magnetic disk. Typically, the ferromagnetic material of the flux guide is required to be stabilized by hard bias layers on each side of the flux guide. With submicron track widths, such as 0.1 xcexcm to 0.2 xcexcm, the hard biasing of the flux guide renders the magnetization of the flux guide too stiff to adequately respond to flux signals from the rotating magnetic disk. Therefore, an alternative longitudinal bias scheme is needed to maintain the sensitivity of the sensor.
To enhance the sensor""s permeability, an alternative longitudinal bias scheme is employed in CPP sensors, namely an in-stack longitudinal bias stack (LBS) which is located at either the top or the bottom of the sensor. The hard bias layer at each side surface of the sensor has been eliminated by providing a bias stack in the sensor stack for longitudinally biasing the free layer of the sensor. The bias stack may comprise a metallic spacer such as Ta and a ferromagnetic longitudinal biasing layer (LBL). The LBL may be a hard biasing layer or a ferromagnetic pinned layer which has its magnetic moment pinned by exchange coupling with an antiferromagnetic (AFM) pinning layer. The metallic spacer may be an antiparallel (AP) coupling layer such as ruthenium (Ru) or tantalum (Ta). In a sensor where the free layer is located closer to the top of the sensor than to the bottom of the sensor the bias stack is located at the top of the sensor and in a sensor where the free layer is located at the bottom of the sensor the bias stack is located at the bottom of the sensor. The invention also applies to a CPP spin valve sensor wherein a nonmagnetic electrically conductive spacer layer is employed in lieu of the barrier layer. The spacer layer is typically made of copper (Cu). The present invention also includes several unique methods of making the sensor, depending upon whether the sensor is a top located free layer type of sensor or a bottom located free layer type of sensor.
An object of the present invention is to improve the longitudinal biasing of a free layer in a current perpendicular to the planes (CPP) read sensor and prevent possible shorts between free and pinned layers during lapping.
Another object is to obviate the stiffening of side portions of a free layer so that the free layer can be provided with a narrow track width for improving the track width density of the read head.
A further object is to provide methods of making the read head with a flux guide and longitudinal biasing of the free layer.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.